(1) Field of the Invention
This invention relates to an electromagnetic variable delay line combining an inductance element and capacitors, and more particularly to improvements in and concerning a variable delay line capable of finely changing the delay time in ultra-high speed signals.
(2) Description of the Prior Art
Among the variable delay lines developed to date for changing the delay time in ultra-high speed signals, involving an ultra-high speed rise time of the order of not more than 1 ns, there are counted those of recent development which are constructed as described below.
The structure involves space coiling a conductor in the shape of a single-layer solenoid to form an inductance element, connecting a plurality of capacitors between the inductance element and ground to form an electromagnetic delay line consisting of a plurality of sections, disposing a conductive plate along the delay line in a manner opposed thereto, disposing a movable contact between the conductive plate and the delay line, held resiliently in contact with the conductive plate and the conductor, and adapting the movable contact to be freely movable in the axial direction of the delay line.
In the variable delay line of the foregoing description, the length of the conductor defined by the movable contact can be changed by moving the movable contact as required. When an ultra-high speed signal is fed to the conductive plate, for example, this signal is applied through the movable contact to the delay line and fed out with an interval of delay time proportionate to the number of sections of the delay line falling between the points of input and output. By moving the movable contact, therefore, the delay time of the output signal can be finely changed.
With the conductive plate adapted such that the movable contact remains constantly in contact with the conductive plate even during its movement, however, since this conductive plate is disposed face to face with the delay line throughout the entire length thereof, there is inevitably formed a stray capacitance between the inductance element of the delay line and the conductive plate, throughout the entire length of the conductive plate.
In the actual manufacture of the variable delay line, the conductive plate is liable to be formed as a relatively large element because it is desired to be easily supported in position. The movable contact is liable to be formed as a small sized element because it is required to permit desired changes in the delay time of ultra-high speed signals without entailing any loss. Consequently, the space between the inductance element and the conductive plate tends to become excessively narrow and the stray capacitance too large to be ignored. When the stray capacitance increases beyond a certain level, it begins to exert very serious adverse effects upon the delay characteristics of the delay line, such as distortion of the waveform of the output pulses from the variable delay line and retardation in the rise time.
These effects which the stray capacitance exerts upon the delay characteristics are aggravated when the adjustment is made in the direction of increasing the delay time, i.e. by moving the movable contact away from the output terminal of the delay line. Thus, the delay characteristics are varied by the change in the delay time. There are times when use of an inductance element obtained by winding a conductor around a flat bobbin proves desirable in the sense that the inductance element will be more easily fixed in position. In this case, the adverse effects of the stray capacitance become more prominent because the conductive plate and the conductor are opposed to each other over a much larger area.